Benthic cyanobacterial mats

MSc thesis

Shifts of coral reefs towards alternative states occur due to local and global stressors. Although global stressors are expected to increase due to climate change, anthropogenic local stressors can be addressed to prevent the loss of important ecosystem services. Identifying and understanding how human activities affect the dynamics in the benthic communities in the reef ecosystems could facilitate more effective reef restoration efforts. But how do human activities affect water quality and subsequently the benthic cover? To answer this question we look towards the coral reefs of Bonaire, home to one of the most pristine reefs in the Caribbean. We combine existing data on human activities and environmental variables with new temporal water quality and benthic cover data generated along the west-coast of Bonaire. We created two sets of models: relating the benthic cover to water quality and explaining water quality with human activity. Because our data collection extended into unexplored deeper parts of the reef we have a unique opportunity to consider the effect of local stressors along a more extensive depth gradient.

We hypothesized that areas with high nutrient loads would be reflected by benthic cover with relatively high algae, sponges and benthic cyanobacterial mats. Our results showed this to be the case for sponges and turf algae, but not for benthic cyanobacterial mats (BCM) and macroalgae. The coral and crustose coralline algae (CCA) cover were expected to be related negatively to the selected water quality variables. The models predicting the coral cover give a mixed result. Both significant positive and negative effects of nutrients on the coral cover have been found, and relatively the positive effects are stronger than the negative effects. The expectation that an increase in human activity leads to a decrease in the water quality is a lot more nuanced, but it is clear that terrestrial human activity plays an important role. The influence of depth on the effects of the water quality on the benthic covers seems to be minimal at most. As the few significant differences in water quality effects found, were more likely to be the effect of under sampling than anything else. However, these results might change as more data becomes available, narrowing both the prediction and confidence intervals and thus increasing the chance of finding significant effects of water quality on the benthic cover and clearer effects of human activity. 

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

Cyanobacteria presence in the marine ecosystem is biologically significant due to its versatile nature. This phylum is responsible for destructive red tides and black band disease as well as building up the limestone in reefs. Nodularia cf. spumigena has been an indicator of groundwater off of the coast of Bahamas and may provide insight into where Bonaire‟s groundwater enters the marine ecosystem. Bonaire is a tropical oceanic island with a fringing coral reef located in the south Caribbean Sea and, although the population of the island is small (~16,000), it has developed a population center of residential and commercial use in Kralendijk, a coastal city. To compare how the benthic cyanobacteria mats off of Bonaire relate to this growing population, a three-fold study was conducted. A large scale pattern of cyanobacteria was studied at nine sites on the leeward side, a medium scale pattern was studied at four sites off the coast of Kralendijk in between two known nutrient outputs, and tests of mean levels of Escherichia coli and total coliforms inside and outside the cyanobacteria mats were completed. Video transects were used to determine percent cover of the large and medium scale patterns and IDEXX technology was used to test pore water inside and outside the mats for two types of bacteria associated with human waste. No definite patterns of groundwater or population center were directly linked to benthic cyanobacteria cover. The large scale did show a higher average cyanobacteria cover throughout the three-year study, indicating that there may be a relationship between the hydrology of the island and nutrient circulation. The medium scale showed an inverse relationship between turf algae and cyanobacteria.

This student research was retrieved from Physis: Journal of Marine Science XII (Fall 2012)19: 9-15 from CIEE Bonaire.

Abstract

Benthic cyanobacterial mats (BCMs) are increasing in abundance on coral reefs worldwide. However, their impacts on biogeochemical cycling in the surrounding water and sediment are virtually unknown. By measuring chemical fluxes in benthic chambers placed over sediment covered by BCMs and sediment with BCMs removed on coral reefs in Curaçao, Southern Caribbean, we found that sediment covered by BCMs released 1.4 and 3.5 mmol C m(-2) h(-1) of dissolved organic carbon (DOC) during day and night, respectively. Conversely, sediment with BCMs removed took up DOC, with day and night uptake rates of 0.9 and 0.6 mmol C m(-2) h(-1). DOC release by BCMs was higher than reported rates for benthic algae (turf and macroalgae) and was estimated to represent 79% of the total DOC released over a 24 h diel cycle at our study site. The high nocturnal release of DOC by BCMs is most likely the result of anaerobic metabolism and degradation processes, as shown by high respiration rates at the mat surface during nighttime. We conclude that BCMs are significant sources of DOC. Their increased abundance on coral reefs will lead to increased DOC release into the water column, which is likely to have negative implications for reef health.

Abstract

Benthic cyanobacterial mats (BCMs) are impacting coral reefs worldwide. However, the factors and mechanisms driving their proliferation are unclear. We conducted a multi-year survey around the Caribbean island of Curaçao, which revealed highest BCM abundance on sheltered reefs close to urbanised areas. Reefs with high BCM abundance were also characterised by high benthic cover of macroalgae and low cover of corals. Nutrient con- centrations in the water-column were consistently low, but markedly increased just above substrata (both sandy and hard) covered with BCMs. This was true for sites with both high and low BCM coverage, suggesting that BCM growth is stimulated by a localised, sub- strate-linked release of nutrients from the microbial degradation of organic matter. This hy- pothesis was supported by a higher organic content in sediments on reefs with high BCM coverage, and by an in situ experiment which showed that BCMs grew within days on sedi- ments enriched with organic matter (Spirulina). We propose that nutrient runoff from urban- ised areas stimulates phototrophic blooms and enhances organic matter concentrations on the reef. This organic matter is transported by currents and settles on the seabed at sites with low hydrodynamics. Subsequently, nutrients released from the organic matter degra- dation fuel the growth of BCMs. Improved management of nutrients generated on land should lower organic loading of sediments and other benthos (e.g. turf and macroalgae) to reduce BCM proliferation on coral reefs.